-Arkanoid.cpp: Set YM2149 emulation to mix channels the same way the
real pcb mixes them, by directly tying them together with no mixing
resistors. This introduces some distortion, but is more accurate
to actual pcb audio. [Lord Nightmare]

-arkanoid.cpp: Added emulation of the 68705 timer and timer interrupts.
Fixed bugs with MCU port c ddr handling, latches and edge detection,
added missing interrupt on Z80 write to MCU, and removed a boost
interleave hack which was made unnecessary by these fixes. Switched
the Arkanoid (older) sets to use original Taito A75 06 MCU code as
opposed to the bootleg MCU code used until now. The 3 (newer) and 1
Tournament sets still use hand-hacked bootleg MCU code until original
MCU chips can be dumped and/or decapped. All 9 of the dumped Arkanoid
MCU code roms now work, if hooked up to the appropriate drivers. Hooked
the now-working 'deprotected Taito' MCU dump up to arkanoidjbl. Added an
alternate gfx rom dumped from an original 'older' US board. [Lord Nightmare, Brizzo]

-some fast invaders improvements [ANY]

-Return std::string objects by value rather than pass by reference [AJR]
- strprintf is unaltered, but strformat now takes one fewer argument
- state_string_export still fills a buffer, but has been made const
- get_default_card_software now takes no arguments but returns a string

-marywu: Most of the hardware has been mapped by analysing the pcb tracks. [Felipe Sanches]
This set of commits include:
a video layout with all 30 LEDs 13 double-digit 7seg modules
an improved memory map hooking up the couple AY8910-3 chips, 2kb of Static RAM and the keyboard/display controller (an i8279 clone)
multiplexing signals for the 7seg displays
LEDs being controlled by the IO ports of the 2 AY8910-3 chips
8-bit set of DIP-Switches, a 2x16 keyboard and 4 push buttons mapped to the i8279 scanlines and return lines

-Some more validity checking improvements: [AJR]
* The -validate command now accepts an optional string, validating only
matching drivers. This has proven useful for debugging. The default is
to validate all drivers as usual.
* Devices' names are tracked when validating their auto-finders.